Field of the Invention
Embodiments of the present invention relate to a control device for an internal combustion engine. More particularly, embodiments of the present invention relate to a control device that is suitable as a control device for an internal combustion engine for use in a vehicle having an operation mode that uses lean combustion.
Background Art
An internal combustion engine equipped with an NOx storage-reduction catalyst in an exhaust passage is disclosed in Japanese Patent Laid-Open No. 2000-170573. The aforementioned internal combustion engine can operate in a lean mode that makes the air-fuel ratio of an air-fuel mixture a lean air-fuel ratio. The exhaust gas of an internal combustion engine contains NOx. The NOx concentration of the exhaust gas is determined by the air-fuel ratio of the air-fuel mixture, and becomes to be a high value in an air-fuel ratio region on a lean side relative to the theoretical air-fuel ratio.
More specifically, the NOx concentration in exhaust gas reaches a maximum at an air-fuel ratio that slightly exceeds the theoretical air-fuel ratio, that is, at an air-fuel ratio of about sixteen, and as the air-fuel ratio increases (becomes leaner) from that value, the NOx concentration in the exhaust gas decreases while maintaining a high value. Therefore, when operating an internal combustion engine in a lean mode, it is necessary to prevent the release of NOx into the atmosphere.
The NOx storage-reduction catalyst that the aforementioned internal combustion engine is equipped with can store NOx that is contained in exhaust gas, within the range of the storage capacity of the NOx storage-reduction catalyst. Consequently, the internal combustion engine can prevent the release of NOx into the atmosphere during operation in the lean mode.
The aforementioned internal combustion engine integrates NOx storage amounts that are stored in the NOx storage-reduction catalyst, and when the storage amount reaches a determination amount, it controls the air-fuel ratio of the air-fuel mixture to become temporarily enriched. Hereunder, the control is referred to as a “rich control”. When the NOx storage-reduction catalyst receives a supply of exhaust gas having a rich air-fuel ratio, the NOx storage-reduction catalyst releases the stored NOx. Therefore, according to the aforementioned internal combustion engine, regeneration of the NOx storage-reduction catalyst can be performed before the NOx storage-reduction catalyst reaches a state of NOx saturation. NOx that is released accompanying execution of the rich control is reduced by HCs and the like contained in the rich exhaust gas, and is released into the atmosphere in a purified state. Therefore, according to the aforementioned internal combustion engine, release of NOx into the atmosphere can be continuously prevented without impairing the exhaust gas purification characteristics.